Sea ice and ocean

Reasonably accurate and consistent assessments of sea ice extent have been available since 1979 based on passive microwave brightness temperatures (see Chapter 7). These records show several interesting features (Figure 11.3). First, there is considerable month-to-month variability in normalized departures. Second, there is evidence of a multiyear cycle. There is also a long-term downward trend. Cavalieri et al. (2003) calculate a trend of about -3.0% per decade. Closer inspection of the sea ice record indicates that the downward tendency is driven primarily by summer and early autumn reductions. As reviewed in Chapter 7, extreme minima were observed in September of 1990, 1993, 1995, 1998, 2002, 2003 and 2004, with 2002 setting a new record low for the passive microwave era. Another notable event during the period 2000-2 was the rapid breakup of the Ward Hunt Ice Shelf north of Ellesmere Island following a steady retreat during the twentieth century (Mueller et al., 2003). Records from ships and land stations provide some evidence of century-scale reductions in sea ice over the North Atlantic (e.g., Walsh et al., 1999), but the quality of the data is open to question.

Considerable attention has been paid to submarine sonar records. Using such data adjusted to account for sampling between different seasons, Rothrock et al. (1999) find that reductions in ice extent have been attended by significant thinning of the perennial ice cover. Comparisons between sea ice draft data acquired during submarine cruises between 1993 and 1997 with earlier records (1958-76) indicate that the mean ice draft at the end of the summer melt period decreased by 1.3 m in most of the deepwater regions of the Arctic Ocean (Figure 11.4). Questions then arose regarding how much

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Year

Figure 11.3 Time series of Arctic monthly sea ice extent anomalies (referenced to the mean for each month), 12-month running anomalies and least squares fit, based on data from 1979 to 2003 (courtesy of J. Stroeve, NSIDC, Boulder, CO).

Figure 11.4 Mean ice draft from submarine sonar data (September-October)within regions for which co-located measurements are available from early cruises (1958-75) and cruises in the 1990s. Data are seasonally adjusted to 15 September to account forseasonal variability(cour-tesy NSIDC, Boulder, CO; based on Rothrock et al., 1999).

Chukchi Beaufort Canada Cap Sea Basin (5) (5) (6)

Eastern All Arctic Regions (2) (29)

Figure 11.4 Mean ice draft from submarine sonar data (September-October)within regions for which co-located measurements are available from early cruises (1958-75) and cruises in the 1990s. Data are seasonally adjusted to 15 September to account forseasonal variability(cour-tesy NSIDC, Boulder, CO; based on Rothrock et al., 1999).

Regions 11993-1997

of this apparent dramatic thinning represents the effects of melt/reduced ice growth, a wind-driven redistribution of thicker ice to along the coast of the Canadian Arctic Archipelago (Holloway and Sou, 2002) (see Chapter 7) or other factors, such as an enhanced flux of thick ice through Fram Strait. The emerging view (see Section 11.4.4) is that all of these factors are likely involved. The updated analysis of Rothrock et al. (2003), based on observations and models, gives further evidence of thinning in the late 1980s through 1997. Thinning finds further support in the ice-ocean modeling studies of Hilmer and Lemke (2000), Rothrock and Zhang (2005) and Lindsay and Zhang (2005). Regarding related oceanic changes, comparisons between data from oceanographic cruises in the 1990s and earlier climatologies indicate that the influence of Atlantic water at 200-900 m depth has become increasingly widespread (e.g., Morison et al., 1998). Polyakov et al. (2004) examined variations in the intermediate Atlantic water layer for the period 1893-2002. Warming from the late 1980s through the late 1990s stands out, but Atlantic layer temperatures exhibit strong low-frequency variability. While hydrographic data are sparse for the earlier part of the record, there appears to have been another warm period from the late 1920s to 1950s and two cold periods, one from the beginning of the record to the 1920s, and another in the 1960s and 1970s. These variations show broad consistency with available time series of SAT, as well as other records such as sea ice extent and fast ice thickness along the Arctic Siberian seas.

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